Replication-defective RNA tumor viruses exhibiting both in vitro and in vivo transforming activity have been isolated from a number of mammalian species. Such viruses appear to represent genetic recombinants between nondefective "leukemia" or "helper" viruses and the host cell from which the transforming sequences were derived. In order to systematically analyze properties of these transforming sequences, experimental efforts have been directed towards isolating a variety of cell species expressing viral coded transforming properties, but nonproductive of intact virions. These lines are used to derive mutant cell lines limited in their expression of transformation. These various cell systems are utilized to generate various antiserums which are then employed for identification of transformation specific proteins.